The invention relates to saw blades and especially to a cutting edge configuration for a saw blade.
It has been long known that for the cutting edges of tools, such as saws, there is an optimal degree of sharpness, as expressed by the radius measured in a cross section through the cutting edge. The optimal radius depends on the material to be cut, on the material of the cutting edge, on the depth of cut and on the cutting edge geometry.
Too large of a radius will generate an excessively large cutting force and irregular chips, which in turn will produce irregular surfaces with bad precision. Too small of a radius will make the cutting edge vulnerable to fracture and cracking, and the edge will vibrate, producing scratches and cracks in the cut surface. An optimal radius will, when cutting metals, allow the formation of a small built-up edge from the cut material around the actual edge, protecting the latter from wear and cracking. Normal values for the edge radius are 15-60 microns for cutting of metals. If the edge of a fresh tool is sharper than that, it will need a running-in period, during which an edge rounding will gradually be produced by wear. During the running-in period the cutting must be done with reduced feed.
Another way of reducing edge cracking is to ensure that the direction of the chip flow is well defined and continuous. If two flows of cut material meet while traveling in different directions, there will occur a very high and fluctuating local pressure, which will cause damage and local erosion and cratering of the rake face near the edge. This occurs where two cutting edges meet at a corner.
The above-outlined principles will now be explained with reference to FIGS. 1A-4. FIG. 1A shows part of a conventional saw blade body 11 with teeth 9, where the teeth are provided with cutting tips 10 formed of a wear resisting material such as cemented carbide. Some of the teeth may be set and some teeth may be straight (unset). The tips are brazed or welded onto the blade body. Alternatively, the tips could be integral parts of the blade body, or separate inserts held in seats in the blade body. Each tip has a rake face 12, two side faces 13 (only one side face visible in FIG. 1), and a clearance face (not visible) projecting beyond the respective surfaces of the tooth 9. A grinding step is then performed on those faces to make them flush with the respective tooth surfaces, as shown in FIG. 1B. As the result of the grinding, the rake face 12 is intersected by the clearance face to form a main cutting edge 14, and is also intersected by the side faces to form a pair of side cutting edges 15. The side faces 13 intersect the clearance face to form a pair of clearance edges 17. If the teeth are to be set, a setting operation is preferably performed after the grinding step.
Most of the cutting effort and the cutting force occurs on the main edge 14. It has been learned that if that edge is too sharp, it may possess micro-cracks and similar defects which can result in premature deterioration of the cutting edge.
Moreover, an overly sharp main cutting edge produces another disadvantage in the case of a flexible blade, such as a band saw blade. That is, a main cutting edge on a band saw blade which is too sharp will tend to pull (feed) into the workpiece (i.e., the blade becomes self-feeding), as permitted by the flexibility of the band saw blade. However, once the main cutting edge has been pulled-in to a certain extent, the band saw blade becomes tensioned and snaps back out of the work, thereby setting up a vibration which can damage the main cutting edge.
The above-described problems relating to an overly sharp main cutting edge can be prevented by rounding-off the main cutting edge. That is, it has been shown that in order to attain smooth cutting and longer life of the cutting tip, the main edge should be slightly rounded, e.g., by abrasive rounding methods such as abrasive brushing or blasting. This will remove micro-cracks and similar defects which occurred during manufacture of the tip, and provide a rounded area where a small and stable built-up edge will form during cutting.
However, the abrasive rounding step has additional consequences that are beneficial in some cases and disadvantageous in other cases. That is, the abrasive rounding procedure serves to round-off not only the main cutting edge, but also the portions of the side cutting edges 15 and the clearance edges 17 situated adjacent the ends of the main cutting edge 14, as can be seen in FIGS. 1C and 1D. In effect, the corner J formed at the junction between those edges 14, 15 and 17 becomes rounded, i.e., almost spherical in shape. When a tooth tip according to FIG. 1D is penetrating workpiece 10, as shown in FIG. 2, a chip C will be formed by the main edge 14 which travels parallel to the side edges. Due to the roundness of the corners J, corner chips C' will also be formed at the respective corners, which travel corner chips diagonally inwardly. Where those chip flows C,C' meet, there will occur a local pressure peak which generates enough frictional heat to break down the rounded corners. Furthermore, the thickness of each corner chip C' is not uniform; rather, the portion of the corner chip produced at the upper end of the corner is much thinner than the lower portion of the corner chip and is thus unable to absorb as much heat. As a result, more of the heat must be absorbed by the cutting tip.
The local pressure peaks can also deflect, deform and merge the chip flows to create a non-flat chip having a generally V-shaped cross-section, i.e., along a center line CL of the chip there is formed a peak. In cutting operations where the depth of cut is large, e.g., in the machining of grooves, that is an advantageous occurrence, because the chips will be narrow and straight-flowing, allowing them to flow easily out of the groove being cut, which is often so important that the extra wear due to the pressure peaks 31 will be accepted.
However, in other cases, i.e., where the depth of the cut is shallower, such as in the sawing of metals or in the use of band saw blades, it is preferred to have flat chips which will form compact curls capable of being carried out of the cut by the saw teeth. Thus, in those cases, heat, wear and chip deformation caused by the presence of local pressure peaks cannot be tolerated.
In the case of band saw blades, then, the abrasive rounding is not performed. Instead, the blade undergoes a running-in period wherein the blade is made to cut continuously for numerous hours until the main cutting edge has been sufficiently worn that the above-described self-feeding tendency is overcome. That running-in procedure, however, is costly and time consuming.
An object of the present invention is to provide a shape of a cutting edge that will ensure a unique chip flow to reduce the edge wear near the corner, and a method to produce such an edge shape, which avoids the need for an extended running-in period. Tools which could be provided with this type of edge include a band saw, a circular saw, a parting or grooving tool, or an insert to be used in such a tool.